Galveston Bay is an estuary, a semi-enclosed coastal water body that has a free connection with the open sea. In an estuary, freshwater from rivers and streams mixes with sea water. Freshwater enters Galveston Bay from the Trinity River (which supplies more that half of the bay's freshwater), the San Jacinto River, and the area's numerous smaller streams and bayous. The bay depends on freshwater inflows to dilute the salty water entering the bay from the Gulf of Mexico. The dilution of salty gulf waters with freshwater inflows is critical to the survival of young fish and shellfish, particularly oysters. Freshwater inflows ensure that the wetlands surrounding the bay remain healthy. Wetlands are important nurseries for the young of recreationally and commercially important finfish and shellfish. A decrease in the freshwater reaching the bay would likely change its salinity and may alter its ecology. Freshwater inflows also carry important nutrients and sediments to the bay. Both are essential in supporting plant communities and sustaining marshes. Periodic droughts affect the salinity of Galveston Bay. Drought conditions are usually short-lived and are relieved by periods of intense rainfall, such as those caused by tropical storms.